Improving the cellular uptake of oligonucleotide analogues represents a critical issue with respect to the application of these biomolecules in the targeting of mRNAs or in the formation of triplex structures with genomic DNA as a means to control gene expression. Our approach entails the systemic tailing of oligonucleotides with monomeric polyethylene glycols at either the 3'-end, 5'-end or both ends. Dr. Koga and Judith B. Regan have recently coupled various polyethylene glycol macromolecules (M.W. 2,000-10,000) onto a controlled-pore glass support (CPG) and have synthesized oligonucleotides from these derivatized supports. This approach led to the facile preparation of oligonucleotide-polyethylene glycol conjugates. All attempts to couple polyethylene glycol macromolecules at the 5'-end of oligonucleotides anchored to CPG, failed. The above conjugates had characteristic mobility on polyacrylamide gel electrophoresis and have not affected hybridization with unmodified complementary oligonucleotides. The Tms of the modified duplexes were similar to that of the native DNA duplexes. To facilitate the monitoring of the cellular uptake of such conjugates, Judith B. Regan has synthesized a fluorescent marker that will be inserted between the polyethylene glycol and the oligonucleotidic moieties of the conjugates. In addition to be applied to the study of cellular uptake, PEG- oligonucleotide conjugates will also be hybridized to large (500 bp) complementary mRNAs to evaluate in vitro, the effect created by the steric hindrance of the polyethylene glycol tail on the translation of these mRNAs relative to proper controls.